DPA is widely used in post-harvest treatments as an anti-oxidant to protect fruit such as apples and pears during long cold-storage against the physiological disease, scald. DPA also has fungicidal activity. It is often formulated as an emulsifiable concentrate which, after dilution with water, can be used as such or in combination with waxes by dipping the fruit into the liquid to cover it with a protective layer.
"Technical grade" DPA available commercially contains a number of by-products (such as acridine, aniline, quinoline, indole, 4-ABP, and 2-ABP) which are inherent to the manufacturing process. Processes for manufacturing DPA are taught in the literature; for example, Kirk-Othmer's Encyclopedia of Chemical Technology gives an overview of such processes in the section describing manufacture of aromatic diarylamines. Several of the aforementioned by-products are of toxicological concern, especially 4-ABP which is a recognized carcinogenic compound. Aniline and 2-ABP are also suspected in this respect. It is therefore important to remove these impurities to a level as low as possible to reduce the risk of human health when consuming fruits thus treated with DPA.
While purification of technical grade DPA can be achieved by well-known techniques such as distillation or re-crystallization, these methods involve high energy costs for heating and cooling. It is an aim of this invention to find a more efficient method for selective removal of the impurities of toxicological concern. Guidelines for these impurities are under review by the World Health Organization, but it is expected that guideline impurity limits will be set at a maximum of 0.5 milligrams/kilogram ("mg/kg") for 4-ABP and a maximum of 2 mg/kg for each of 2-ABP and aniline. Thus, a further aim of this invention is to find a method for preparing a purified, food grade quality, DPA whose level of aniline, 2-ABP and 4-ABP are well below the guideline limits.
Applicants are not aware of any literature disclosing the use of ion exchange resins to achieve DPA purification.